Convert to pytest

[gadomski]

As mentioned in stac-utils/stactools#218 (comment), PySTAC is currently not using pytest, but it probably should.

This could be as simple as changing over the requirements-test.txt and the testing scripts, but it could also include more in-depth updates to use pytest features. In particular, pytest-vcr could speed up/robustify network tests (we get occasional CI errors from 500s from CMR, e.g.: https://github.com/stac-utils/pystac/actions/runs/3434301808/jobs/5725483518).